Many different types of fluid flow measuring devices are used commercially. A common type of flow measuring device is a head meter which operates by relating the pressure drop through a flow restriction to either the point velocity (pivot tube) or average velocity (orifice, venturi) via a mechanical energy balance.
With a cavitating venturi, the flow therethrough is proportional to the square root of the difference between the inlet pressure to the device and the fluid saturation pressure. Thus, for systems which provide a constant value for this pressure difference, the use of a cavitating venturi will result in a precise measurement and control of the flow rate. However, if heat losses occur in the system, such as through the plumbing of the system, the saturation pressure rather than remaining constant will decrease over time, resulting in a higher value for the relevant pressure difference. This in turn will result in the device measuring a higher flow rate therethrough.
Such a result is very undesirable in systems such as two-phase thermal management systems which have one or more evaporators connected in parallel and controlled by cavitating venturis. Substantial heat losses in such a system will result in increased flow rates which in turn assembly will change the pressure delivered by the flow control assembly and generally upset the flow control in all parallel branches.
A varying value for the difference between the inlet pressure to the device and the fluid saturation pressure, as realized by systems wherein heat loss occurs, significantly increases the difficulty of using cavitating venturi devices for flow measurement and control systems.